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Tuesday, February 26, 2013

Mediterranean diet cuts risk of heart disease!

This story is making headline news yet again, including at the New York Times, where Gina Kolata describes a new study done in Spain of the effects of adding extra virgin olive oil or nuts, fish, fruits, legumes and red wine to the diet, and reducing the consumption of red meat and processed baked goods. (Note that the pyramid below was adapted from Consumer Reports Nov 1994.)

The total study group of about 7000 men (aged 55-80) and women (aged 60-80) was divided into 3 subgroups, and asked to follow the Mediterranean diet (MD) with extra virgin olive oil, the MD with nuts, or a low-fat diet. They weren't asked to lose weight or to exercise. The MD groups were given olive oil or nuts every week, and counseling as to how to follow the diet, while the low-fat diet group (the control group) was given a pamphlet on how to follow the diet when enrolled in the study, and then annually until 2006, when it was recognized that their adherence to the diet was poor, at which point researchers added further intervention. This group still never could consistently follow the low-fat diet, and instead were essentially eating their usual diet.

Recruits were people without heart disease but with type 2 diabetes, or at least three major risk factors ("smoking, hypertension, elevated
low-density lipoprotein cholesterol levels, low high-density lipoprotein
cholesterol levels, overweight or obesity, or a family history of
premature coronary heart disease"). They were followed up until either they dropped out or until the year 2010. The total person-years in the study was about 12,000, 11,000 and 10,000, by group.

The paper is published in the New England Journal of Medicine. The researchers assessed primary and secondary outcomes, with the former being heart attack, stroke or death from cardiovascular disease, and the latter being heart attack, stroke, death from cardiovascular disease or death from any other cause.

The median follow-up period was 4.8 years. A total of 288
primary-outcome events occurred: 96 in the group assigned to a
Mediterranean diet with extra-virgin olive oil (3.8%), 83 in the group
assigned to a Mediterranean diet with nuts (3.4%), and 109 in the
control group (4.4%). Taking into account the small differences in the
accrual of person-years among the three groups, the respective rates of
the primary end point were 8.1, 8.0, and 11.2 per 1000 person-years. Outcomes According to Study Group.).
The unadjusted hazard ratios were 0.70 (95% confidence interval [CI],
0.53 to 0.91) for a Mediterranean diet with extra-virgin olive oil and
0.70 (95% CI, 0.53 to 0.94) for a Mediterranean diet with nuts.

And,

In this trial, an energy-unrestricted Mediterranean diet supplemented
with either extra-virgin olive oil or nuts resulted in an absolute risk
reduction of approximately 3 major cardiovascular events per 1000
person-years, for a relative risk reduction of approximately 30%, among
high-risk persons who were initially free of cardiovascular disease.

There was no effect of diet on mortality from all causes. That is, the
difference in total number of deaths between groups was not
statistically significant. The effect of diet on cardiovascular disease was apparently through stroke, not heart attack. People following the Mediterranean diet did not lose weight, nor reduce the amount of fat in their diet, so the effect, the researchers say, was of dietary components alone.

Interestingly, when researchers compared the merged MD groups with controls recruited before and after 2006, they found that "adjusted hazard ratios were 0.77 (95% CI, 0.59 to 1.00) for participants
recruited before October 2006 and 0.49 (95% CI, 0.26 to 0.92) for those
recruited thereafter (P=0.21 for interaction)." Remember that the researchers decided to do more intervention with the control group after 2006.

But how is that to be interpreted? Were controls adhering better to the low-fat diet after 2006, and it turns out to be significantly worse than the MD? Or vice versa? Did the act of intervention itself made a difference somehow, or were the people recruited after 2006 metabolically different, older, sicker or something else from those recruited before? Whatever the reason for the difference, it does suggest that comparison between the three groups is not a simple comparison of three different diets.

If this study is as significant as the write-ups are saying, it means that the effect of changing diet can be significant enough to be detectable within a relatively short time span (this study spanned 2003-2010, with subjects apparently included for varying lengths of time). This suggests that the effect of a non Mediterranean diet over a lifetime is reversible, and thus that risk isn't as genetic as some think (not a surprise!), risk factors like blocked arteries may be reversible by diet, cholesterol can be changed by diet (this is also well-known), being overweight is not a significant risk factor (results on this issue go back and forth), and low-fat diets aren't protective (this, too, has been shown before, though doesn't seem to have caught on with the public).

And, while this study does confirm things that have already been known, actually rather well and for quite a long time, it also means that people on the Mediterranean diet still die of cardiovascular disease, particularly heart attacks. Rather than 11 CVD deaths per 1000 person-years, there were 8. So, the difference may be statistically significant, but it's not qualitatively huge, like 25 vs 2, or even 11 vs 2. Though, of course if you're one of those three, that's an incalculable difference. Further, we don't know whether it's eliminating red meat and baked goods rather than adding olive oil and wine and nuts that makes the difference.

One can ask to what extent we should even be doing more and more studies of the same basic idea, once we have systematic data in its favor (dramatic disease benefits were found in a major North Karelia Finland project a long time ago, for example, in a huge dietary intervention study in a place that had, at the time, the highest CVD rates in the world). There are various ways to measure effects and benefits, and to define outcomes, and these are relevant to evaluating any studies of diet and health.

If nothing else, this study is a reminder that if you reduce deaths from one cause, deaths from other causes go up. People do still die of something.

10 comments:

Nice discussion of this paper. Thank you. So, it appears from Table 3 in the paper that "death from any cause" does not differ among treatments. By subtraction, that means that the Med' diet makes you less likely to die from cardio events, but more likely to die from others? If so, I wonder what risks get increased on the diet? Falling drunk into a barrel of olive oil comes to mind but, seriously, the overall lack of effect on total deaths is interesting.

Yes, the diet doesn't seem to be extending life, but rather making room for competing causes. Like drowning in olive oil. And, that does raise the question of why the authors don't propose that the diet in fact _raises_ risk of death from non-cardiovascular causes, which is as likely an interpretation as any other -- something I thought about while drinking my red wine and eating almonds last night.

The commentary on this paper seems to agree that the results are dramatic, but I'm not actually so convinced. Partly because they don't strike me as so dramatic (statistical significance notwithstanding), but also because the study leaves a number of questions unanswered. That's fine, no study can answer all questions, and the Mediterranean diet may be a generally healthy one, as this and other studies suggest. But diet and bodies are complex -- and many delicious pastries are made along the Mediterranean, too.

There is so much evidence already about the basic nature of the Med diet that one hardly needs this study at all.

One very poorly understood problem, even in the professions and certainly by the public, is that of competing causes. Short of guaranteeing immortality, if you reduce one cause of disease and death, you increase that of what's left. Remove heart disease, you increase the rate (though at later ages) of cancer, senility, and so on.

This is a major never-addressed issue in health research today, which is so heavily about late-onset diseases.

The success of all this work could be later, but longer-lasting, more misery-making degenerative diseases. And vastly more health care costs for society to bear.

Was about to say something to the effect of "If you don't die from heart disease, you'll eventually die from cancer" but you seem to have beaten me to it, hehe.

Something of this conundrum reminds me of a query an old history teacher long ago posed to my class: "Even though military helmets were far more protective in World War II than during WWI, there were more injuries in WWII. Why?" The answer was that more people died from head injury in WWI, whereas helmets in WWII did a better job at protecting the head and subsequently there were more head injuries as opposed to fatalities.

The problem of competing causes is not new but is buried in wishful thinking. It goes beyond the disease issues you outline, to deeper issues of cost to the population, total person-days of morbidity and poor-quality life....and the fanciful-comforting-promises industry (known as NIH) that is replacing ministers.

A truly serious discussion would be more realistic about life and death and social well-being and so on. But we can't have such a discussion because it would mean accepting death in a deeper way than hostels do currently.

And, of course, it would threaten the science welfare system (just as science may threaten the minister-welfare system). That's not really true, at least, there _are_ important diseases that really might be cured or prevented by focused, intense research--mainly those of childhood, where decades of quality life might be the harvest.

And we could do a lot of that with lower budgets, spending the savings on lifestyle preventives, which would reap a massively greater harvest of improved qualify of life.....

A few minor comments from a Mediterranean guy working 3 mi. away from that study HQ is Barcelona...

* They discouraged industrial baked goods, because many are chock-full with trans fats. Trans fat awareness is very low in Spain, and they are not requited to be labelled. Only the high-end brands will use (and advertise) olive oil.

* The Mediterranean Diet may be on its way to extinction. Urban dwellers have little time to cook, and processed foods are too popular. Still, there are some reasons for hope. Many children eat their lunches from the school canteens (lunch boxes are rare). Schools follow quite strict dietary guidelines, and often supply recommended diners to complement the lunches that the students have been given. Definitely, Jamie Oliver would be out of his crusade in Spain. Alas, the economic crises has meant cuts in lunch vouchers, and more children are eating home, presumbaly worse than at school.

* And a little bit of self-promotion: with a grad student of mine, we published a few years ago an ecological correlation of the then described genetic risk factors for cardiovascular disease... and it turned out that the correlation was negative. Such risk alleles have higher frequencies in the Mediterranean than in northern Europe! (http://www.ncbi.nlm.nih.gov/pubmed/17683517)

If industrialized diets are allowed to take over, and they are 'non-Mediterranean', then we'll have higher health-care bills in the future to counter-balance the savings we make today by eating factory-food.

I've spent a lot of time in Finland, and in Barcelona where my daughter also lives (a great place!). I like the food in both places. But I think I would last a lot longer in Barcelona than in Helsinki!

Very interesting. Thanks so much for your comments. Rather ironic that in the land of olive oil, olive oil has become high-end. Also, discouraging industrial baked goods because they are chock-full of trans fats does bring up the question -- are the supposed benefits of the MD due to what's added, or what's taken away?

The results you report in your paper are just a perfect confirmation of how risky it is to draw conclusions about any of this. A wry commentary -- from the land of the wry caganer. ;-)

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